Method for treating wastewater

ABSTRACT

A method and apparatus for treating wastewater. The apparatus comprises a reactor having a flow channel generally centrally located in a reactor and immersed in the water or wastewater in the reactor. An agitator is disposed within the flow channel and induces water or wastewater to enter the open upper end thereof and to move downwardly through the flow channel where the water or wastewater is discharged via a flow divider. A reagent is injected into the water or wastewater and the agitator within the flow channel serves to mix the reagent with the water or wastewater passing therethrough and in the process causes at least a slightly turbulent and downward axial flow through the flow channel.

This application is a U.S. National Stage application of PCT ApplicationNo. PCT/FR2004/003323, with an international filing date of December 21,2004. Applicant claims priority based on French application serial no.0315161 filed December 22, 2003.

I—TECHNICAL FIELD OF THE INVENTION

The invention relates to a method and a reactor for the treatment byflocculation of a raw fluid to be treated. The method and the reactorcan be used in particular for treating industrial process water,drinking water and municipal or industrial waste water.

II—PRIOR ART

Various fluid treatments, in particular in the field of water treatment,involve mixing a raw fluid to be treated with a secondary fluid or flowin order to make the secondary flow react with components of the mainfluid; in practice the secondary fluid contains a flocculating agent andit usually contains a granular material on which flocs resulting fromthe action of the flocculating agent on the impurities of the raw fluidto be treated are formed; this process is known as ballasted flocphysical-chemical treatment; the fluids are usually mixed and reacted infull-mixing open reactors equipped with axial-flow vertical-shaftagitators.

In principle a physical-chemical reaction treatment involves dispersingthe secondary fluid or fluids in the raw fluid to be treated andintensive mixing followed by as short as possible a reaction time withstirring at a low intensity (compared to the intensive mixing step),these steps usually leading to the use of a plurality of vats orreactors in series.

The possible existence of a bypass (short circuit) between the vat inletand outlet and the fact that in practice use of the volume of the vat isincomplete, as is easily demonstrated by mathematical modeling linked tophysical models, usually lead to increasing the size of the reactorsand/or to the use of additional agitation power. This induces anincrease in the investment and operating costs, however. Furthermore, itis standard practice to have the inlet and the outlet of a given vat atthe greatest possible distance from each other, for example by using alow-level inlet at one end of the vat and a high-level outlet at theother end, although this proves to be a problem if it is necessary toconnect a plurality of vats in series.

What is more, to prevent rotation of the liquid mass, usually called avortex, which has been recognized as compromising mixing performance, ithas already been proposed to place vertical baffles against the lateralwalls (see “Mixing in the chemical industry”, Sterbacek and Tausk,Pergamon Press, 1965, pp 278-301). According to that document, addingvertical baffles increases turbulence (and therefore improves mixing)provided that the width of the chicanes is from 0.056 to 0.12 times thediameter of the agitator and it is preferable for the baffles to be inthe flow rather than against the walls. In fact the above document alsomentions, for historical reasons, a cruciform baffle formed of twovertical walls mounted on the bottom of the vat, near the agitator, butindicates that the process is then difficult to control and thecirculating liquid rapidly becomes contaminated with solid particles. Italso indicates that these walls induce an increase in power consumption.

In the case of deep vats, it is known in the art to dispose a pluralityof mobiles in the volume of the vat, along the agitation shafts, toincrease the fraction of the volume in which turbulence is induced; ithas nevertheless been realized that these stacked mobiles can bedispensed with by disposing a concentric tube (flow-guide) around themobile, which is mounted at the tube outlet (see the 1965 book citedabove). A flow-guide of this kind acts like an aspiration tube,contributes to increasing the fraction of the volume that is stirred andhas the advantage of allowing internal recirculation. However, thisaspiration effect is often associated with a vortex type rotationmovement.

In the field of water treatment, and in particular with regard to theflocculation step, it has already been proposed, for example in thedocument FR-2 553 082, to produce a reaction chamber with a central areaprovided with an upward axial flow screw and a peripheral area aroundthe central area. The water to be treated, where appropriate with thenecessary reagents added to it, which has been mixed with sludgeobtained previously during the treatment, is introduced at the base ofthe central area, and an additive such as a polymer is introduced intothis area. This produces internal recirculation, the peripheral areaconstituting a slow flocculation area. The central area is delimited bya vertical tube disposed in a parallelepiped-shaped enclosure, and sothe central area may be described as a double area. The mixture thenenters an intermediate enclosure before passing into a separation area.Note that an architecture of this kind involves a large number of areas.

There is also known, from the document WO-98/14258, an installation inwhich a central internal recirculation enclosure includes a central tubecontaining one or more upward axial flow screws, a granular materialinlet and a flocculating agent inlet. The raw fluid to be treated isintroduced at the base of the tube, overflows into the annular area fromthe central area and then, depending on the size of the flocs, riseseither inside the tube, to be recirculated, or in the peripheralclarification area. The screw produces just sufficient turbulence tomaintain the solids in suspension without shearing them, which can leadto slow mixing and therefore degrade performance.

III—TECHNICAL PROBLEMS AND SOLUTIONS ACCORDING TO THE INVENTION

The invention aims to provide an enhanced combination of compactness,efficacy and moderate cost, whether from the plant or operation point ofview.

Accordingly, the invention aims to provide a flocculation reactor which,thanks to one or more agitators and the delimitation of areas withdifferent levels of agitation, improves the kinetics of the reactionbetween a raw fluid to be treated and a flocculating agent (and whereapplicable a granular material forming a flocculation ballast),increases the usable fraction of the volume of the reactor and reducesthe risk of bypassing the process (i.e. the risk of no circulationoccurring in the areas with different levels of agitation), with amoderate level of the power devoted to the agitation levels.

The invention further aims to provide a method of treating a raw fluidto be treated by flocculation and separation, the method combining,within the overall dimensions of a single vat, two areas with differentlevels of agitation, and maximizing the turbulence for a given energylevel and for a given contact time.

A subsidiary aim of the invention is to facilitate the series connectionof a plurality of reactors of the same or different types and of thesame or different sizes, either at the time of manufacture orsubsequently.

To this end, the invention firstly proposes a method of treating a rawfluid to be treated by flocculation and separation charged withimpurities in suspension, colloidal impurities or dissolved impurities,wherein:

the raw fluid to be treated is circulated with a flocculating reagent ina flocculation vat to obtain a flocculated mixture in which theimpurities form flocs, and

-   -   this flocculated mixture is circulated in a separation area in        which the flocculated mixture is separated into clarified        effluent and sludge containing the flocs,

characterized in that:

-   -   a completely immersed flow-guide tube delimits a central area in        the flocculation vat in which agitation (8) brings about        turbulent axial flow of the mixture of the raw fluid to be        treated and the flocculating agent in an axial direction of the        flow-guide tube,    -   that flow is divided (5) angularly by means of a static system        opposing rotation of the flow disposed at the flow-guide tube        outlet,    -   this mixture is allowed to circulate in an opposite direction in        a peripheral area (3) around the central area in order to reach        the inlet of the central area, and    -   a fraction of the mixture is passed to the separation area.

Note that the invention therefore teaches the use of a static deviceopposing rotation of the fluid at the outlet from the flow-guide tube tocombine delimitation of a central area with a high level of agitationfrom a peripheral area with a lower level of agitation with an angulardistribution of the outflow from the central area, so that turbulence ismaximized and dead areas escaping the circulation induced in the volumeof the central and peripheral areas are minimized. As stated above,using a static system to prevent the rotation of a flow had beenabandoned as being difficult to control and had been proposed only incombination with an agitator; the person skilled in the art wouldtherefore have had no reason to think that a static system of this kindcould contribute to solving the stated technical problem. Also, therehas been nothing, even drawing inspiration from prior art solutionsusing a flow-guide in a flocculation area, to suggest that it might bebeneficial to use a static system of this kind in combination with aflow-guide.

According to preferred features of the invention, which may whereappropriate be combined:

-   -   the flow in the central area is maintained at a flowrate from 1        to 20 times the inlet flowrate of the raw fluid to be treated,        which corresponds to sufficient recirculation to reduce the risk        of bypassing the system and to induce sufficient turbulence to        ensure good mixing without necessitating excessive consumption        of energy,    -   the peripheral area is divided into an upstream peripheral area        communicating with the inlet into the flocculation vat for raw        fluid to be treated and a downstream peripheral area        communicating with the flocculated mixture outlet, so as to        cause the raw fluid to be treated that enters the flocculation        vat to enter the central area at least once before passing to        the separation area, which of course contributes to avoiding        bypassing the system without significant consumption of energy        and without involving a large number of recirculation cycles,    -   the turbulent axial flow of the mixture is vertical, which        represents a proven conventional configuration,    -   the vertical turbulent axial flow is brought about by agitation        at mid-height in the central area, which contributes to        obtaining good aspiration movement at the inlet of the central        area and good discharge at its outlet, therefore with an angular        distribution at that outlet, without requiring more than one        agitator,

the turbulent axial flow of the mixture is downward and the mixture isdivided angularly over at least substantially two thirds of the heightbetween the level of the outlet from the central area and the level ofthe bottom of the flocculation vat; it is worth mentioning that this isthe opposite of the direction used at present when there is internalrecirculation in a flocculation area; however, recent modeling studieshave shown that, in contrast to what the person skilled in the art mightthink, it is entirely realistic to use a downward movement incombination with a static angular distribution system without thisinducing unwanted contamination of the latter system by flocs beingformed or growing; selecting a downward direction avoids causingwavelets on the surface as a consequence of the turbulence induced anduses the vat bottom to bring about a rapid change in the direction offlow of the mixture between the central area and the peripheral area.

In the case of downward movement, it is preferable if:

-   -   the mixture is divided angularly over substantially all of the        height between the level of the central area and the level of        the bottom of the flocculation vat, which guarantees an angular        distribution of the whole flow leaving the flow-guide tube,    -   the flow-guide tube delimiting the central area is disposed so        that its bottom end forming its outlet faces the bottom of the        flocculation vat at a distance from ⅓ to ⅔ the average width of        the tube therefrom, which contributes to reversing the direction        of flow with no great risk of unwanted slowing of the flow,    -   the flow-guide tube delimiting the central area is disposed so        that its top end forming its inlet faces the surface of the        contents of the flocculation vat at a distance from ⅓ to ⅔ of        the average width of the tube therefrom, which contributes to        ensuring efficient feeding of the flow-guide tube without        inducing unwanted surface movements,    -   the peripheral area is divided over an upper portion of its        height into an upstream peripheral area communicating with the        inlet into the flocculation vat for the raw fluid to be treated        and a downstream peripheral area communicating with the        flocculated mixture outlet, so as to cause the raw fluid to be        treated entering the flocculation vat to enter the central area        at least once before passing to the separation area, which is a        particularly simple way to obtain the advantages cited above in        respect of the general use of this kind of division,    -   said division is effected over substantially the upper half of        that height, which represents a height sufficient to minimize        the risk of circumventing the central area.

According to other preferred features of the method of the invention:

-   -   the raw fluid to be treated enters and the flocculated mixture        leaves substantially at the level of the inlet area of the        flow-guide tube, which contributes to allowing the series        connection of installations for implementing the method and        participates in efficient guidance of the fluid to be treated        towards the central area and of the mixture that has circulated        in the peripheral area,    -   the flocculating agent is a natural, mineral or synthetic        polymer,    -   the flocculating reagent with which the raw fluid to be treated        is mixed in the flocculation vat is introduced into the fluid to        be treated upstream of said vat,    -   alternatively, the flocculating reagent with which the raw fluid        to be treated is mixed is introduced into the flocculation vat,        for example between the inlet of the flocculation vat and the        inlet of the flow-guide tube; however, it is recommended that        the flocculating reagent is introduced into the central area,        preferably at the boundary of the central area, which ensures        very fast mixing with the fluid to be treated; this effect is        reinforced if at least a fraction of the flocculating reagent is        injected annularly at the periphery of the inlet of the central        area and coaxially with the flow-guide tube,    -   a powder is mixed with the raw fluid to be treated in the        flocculation vat, and is preferably a ballast consisting of an        insoluble granular material heavier than the raw fluid to be        treated, to serve as ballast for the flocs being formed or        growing; the ballast advantageously consists of fine sand with a        particle size range from 20 to 300 microns, the cost price of        which is particularly reasonable,    -   the sludge is treated at the outlet from the separation area,        where ballast is recovered and recycled to the flocculation vat,        which avoids losing the powder in the sludge that is discarded        on reaching the separation area outlet, as well as reducing its        volume,    -   the raw fluid to be treated is mixed with coagulating agent        before it is introduced into the flocculation vat, which        contributes all the more to the efficacy of final separation if        the raw fluid to be treated is water to be treated; in this        case, the water to be treated is preferably mixed with a        coagulating agent including a mineral salt such as an iron or        aluminium salt before it is introduced into the flocculation        vat,    -   separation is effected by sedimentation or flotation,        advantageously with the assistance of separation aid members        such as inclined or vertical tubes or plates; the flocculated        mixture is advantageously introduced tangentially into the        separation area so as to combine a vortex effect with the        sedimentation effect, which improves separation for a given        treatment time or accelerates the treatment for a given level of        separation.

Another aspect of the invention proposes, for implementing a preferredembodiment of the method, a reactor for treating a raw fluid to betreated by flocculation charged with impurities in suspension, colloidalimpurities or dissolved impurities, including a vat having a fluidinlet, a fluid outlet and a flocculation area in a bath in which thefluid to be treated is mixed with a flocculating agent, said reactorincluding:

-   -   a flow-guide tube open at both ends and disposed vertically so        that it is completely immersed in the bath in the vat but        remains at a distance from the bottom of the vat, delimiting a        central area from a peripheral area, the central and peripheral        areas communicating with each other at both ends of the tube and        the peripheral area communicating with the fluid inlet and        outlet,    -   a vertical-axis agitator disposed in the tube so as to generate        therein vertical turbulent axial movement,    -   a cruciform baffle formed of a plurality of vertical walls        extending horizontally from a common edge substantially aligned        with the axis of the agitator, on the downstream side thereof,        so as to divide angularly the flow towards the peripheral area        leaving the tube.

A reactor of the above kind has the advantages described hereinabove inthe case of using the method with a vertically downward turbulentmovement.

According to preferred features of the invention, in some instancesanalogous to those mentioned hereinabove with regard to the method ofthe invention:

-   -   the flow-guide tube has a constant section, which contributes to        ensuring a fast flow of the mixture,    -   the flow-guide tube has a cylindrical shape (in the narrow sense        of the term, namely that its section is a circle), which        represents a particularly simple structure; alternatively, the        section is a regular polygon, etc.,    -   the agitator is disposed substantially at mid-height in the        tube,    -   the flow-guide tube has a diameter from 102% to 120% of the        diameter of the agitator, which ensures good stirring throughout        the section of the flow-guide tube,    -   the hydraulic diameter of the central area is from 40% to 60% of        the average width of the flocculation area formed by the central        area and the peripheral area,    -   the agitator is disposed and driven so as to generate a downward        vertical movement in the tube, the cruciform baffle being        disposed between the bottom of the flow-guide and the bottom of        the vat,    -   the tube has a bottom end facing the bottom of the vat at a        distance from ⅓ to ⅔ of its diameter therefrom,    -   the tube has a top end facing the surface of the bath contained        in the vat at a distance from ⅓ to ⅔ of its diameter therefrom,    -   the distance between the bottom end of the tube and the bottom        of the vat and the distance between the top end of the tube and        the level of the bath are at least approximately 50% of the        diameter of the tube,    -   the cruciform baffle has a height substantially equal to at        least ⅔ of the distance between the bottom end of the tube and        the bottom of the vat,    -   the cruciform baffle has a height substantially equal to the        distance between the bottom end of the tube and the bottom of        the vat,    -   the vertical walls of the cruciform baffle extend over a        horizontal distance substantially from ¾ to 5/4 of the radius of        the tube, which ensures that all or virtually all of the        flocculated mixture is distributed angularly; the vertical walls        of the cruciform baffle preferably extend over a horizontal        distance substantially equal to the radius of the flow-guide        tube,    -   the cruciform baffle comprises four walls offset at 90° around        the axis of the flow-guide tube, which represents a particularly        simple structure (two plates crossing at a right angle),    -   two of the walls are disposed transversely to the direction in        which the raw fluid to be treated enters the flocculation vat,        which contributes to good division of the flow,    -   vertical walls divide the peripheral area between said inlet and        outlet areas over at least a portion of the total height between        the bottom of the vat and the surface of the bath, which helps        to guarantee that the raw fluid to be treated passes through the        flow-guide tube at least once,    -   these vertical walls extend over a vertical distance from 40% to        60% of the total height, which is a good compromise relative to        the height, to ensure good efficiency without using up too much        baffle area,    -   the reactor includes two or more vertical baffles extending        substantially over the upper half of the vat, between the        flow-guide tube, the fluid inlet and the fluid outlet,        respectively, so as to cause the fluid to be treated to enter        the central area between the fluid inlet and the fluid outlet at        least once, because these walls are most effective in the upper        portion, at the level of the flow-guide tube inlet,    -   the vertical walls extend over a height between the main fluid        inlet level and the level of the agitator,    -   the vertical walls extend from the periphery of the peripheral        area to the tube, which ensures good partitioning of the        peripheral area,    -   the inlet and outlet areas are near the level of the surface and        each is provided with a transverse plate facing the inlet and        the outlet, respectively, to form a siphon, which helps to        ensure regular arrival of fluid to be treated and to prevent        unwanted movement on the bath surface,    -   it further comprises a flocculating reagent injection tube        connected to a supply of flocculating reagent,    -   the flocculating reagent injection tube is situated between the        inlet for raw fluid to be treated and the inlet of the        flow-guide tube; the reactor preferably includes an annular        flocculating reagent injection tube coaxial with the inlet of        the flow-guide tube,    -   it further comprises a powder injection tube connected to a        supply of powder,    -   the supply of powder is a supply of fine sand,    -   the vat includes a single flocculation area including a tube,        although the invention also covers situations in which the same        vat comprises a plurality of juxtaposed flocculation areas.

The invention also provides a fluid treatment installation including areactor of the type cited above and a separation area connected to theoutlet from the vat of that reactor.

Preferably:

-   -   the reactor comprises a powder injection tube connected to a        powder supply and the separation area includes an outlet        connected to a powder recovery system and adapted to collect        sludge containing flocs, said powder supply being connected to        said recovery system,    -   the separation area is a sedimentation unit disposed on the        downstream side of the vat,    -   the separation area is a sedimentation unit around the vat.

Accordingly, in a particularly beneficial combination of features of theinvention, the flow-guide tube is disposed above a cruciform baffle, thetwo devices advantageously having at least approximately the samediameter. The recommended pumping direction is downwards, to allow theintroduction of one or more secondary flows or fluids at the surface,thereby ensuring continuous control of the injection thereof. Although,to prevent deposition areas, the person skilled in the art has ruled outhorizontal bottom walls of this kind, the steep velocity gradients thatthe cruciform baffle produces in the lower portion of the vat (inproducing turbulent kinetic energy) have the advantage of preventing theformation of deposition areas.

The combined effect of the above systems is to convert a major portionof the radial component of the screw of the agitator into an axialcomponent, thereby significantly increasing the pumping throughput forthe same absorbed power.

It is worth making the following remarks in relation to preferredembodiments of the invention:

-   -   The presence of two perfectly delimited areas having radically        different mixing intensities means that functions usually        necessitating two vats with different mixing intensities can be        accomplished in the same vat.    -   The use of a single vat having two areas with different mixing        intensities means that the two functions can be implemented with        a single agitator instead of two.    -   For exactly the same usable volume, as a result of the reduction        in dead areas resulting from improved control of mixing        intensity and homogeneity, the total volume of a single vat may        be significantly less than that of two joined vats.    -   For the same treatment efficiency, the same consumption of        reagent and the same pumping throughput, the energy consumption        may be reduced through the conversion of a major portion of the        radial flow into axial flow.    -   In deep vats that normally use a plurality of stacked mobiles,        by eliminating the consumption specific to the additional        mobiles and improving efficiency by using the new design, the        presence of the flow-guide with its associated cruciform baffle        means that the same hydraulic effect can be achieved with a        single mobile and using less electrical power.    -   There may be two vertical baffles filling the space between the        flow-guide and the vat completely and disposed only in the upper        half of the liquid depth; in this case, they are wider and        shorter than indicated in the literature. In the horizontal        plane they are parallel to the inlet walls and therefore        parallel to the outlet walls.    -   The cruciform baffles preferably have a diameter equal to that        of the flow-guide and a height contained between the flow-guide        and the bottom of the vat.    -   By virtue of the advantageous combination of flow-guides,        surface baffles, cruciform baffles, feed and outlet areas and        agitator positioning and secondary fluid injection mode, the        present reactor can achieve an optimum performance improvement.

IV—DESCRIPTION OF THE INVENTION

Objects, features and advantages of the invention emerge from thefollowing description, which is given by way of illustrative andnonlimiting example and with reference to the appended drawings, inwhich:

-   -   FIG. 1 is a diagrammatic perspective view of a preferred        embodiment of a reactor of the invention,    -   FIG. 2 is a diagrammatic plan view of the reactor,    -   FIG. 3 is a diagram of a water treatment installation using the        reactor shown in FIGS. 1 and 2,    -   FIG. 4 is a diagrammatic plan view of a reactor that is a        variant of the reactor shown in FIGS. 1 and 2,    -   FIG. 5 is a diagrammatic plan view of a reactor that is also a        variant of the reactor shown in FIGS. 1 and 2,    -   FIG. 6 is a diagram of one variant of the installation shown in        FIG. 3, and    -   FIG. 7 is a diagram of another variant of the installation shown        in FIG. 3.

Reactor Shape

The reactor 10 shown in FIGS. 1 and 2 is of rectangular shape in orderto facilitate connecting a plurality of reactors of the same type inseries. In a variant that is not shown, the reactor may be square, whichmay be beneficial given the symmetry of the central area (see above).

It should be noted that most features described hereinafter are equallyapplicable to circular reactors (see FIG. 5), which are also within thescope of the invention, the results obtained being practicallyidentical.

The preferred embodiment of the reactor 10 considered here generallycomprises:

-   -   a fluid inlet area 1 receiving a raw fluid to be treated, often        constituting turbulent flow and into which a first reagent, such        as a coagulating reagent of any appropriate type known in the        art, may already have been injected,    -   a high dissipated energy central area 2 inside a tube 2A serving        as a flow-guide for a flow generated by an agitator 8 and in        which the raw fluid to be treated is dispersed and mixed        completely with at least one flocculating reagent; in line with        this tube is a cruciform baffle 5,    -   a low-energy peripheral area 3 outside the flow-guide, in which        the required flocculation is completed,    -   a fluid outlet area 4 that advantageously dissipates the        stirring energy and evenly divides the mixed flocculated fluid,        which here exits across the full width of the vat, in order to        encourage solid-liquid separation in a downstream separator        reactor, if any,    -   a set of vertical baffles 6A and 6B forming deflectors to        minimize bypassing between the inlet and the outlet and making        mixing more efficient.

To facilitate the (initial or later) chaining of a plurality oftreatment steps based on the same principle, the inlet and the outlet ofeach reactor are disposed at substantially the same level, preferably inthe upper portion, substantially at the level of the tube inlet, here atthe surface; this also minimizes dynamic bypassing and enables anyfloating material to be evacuated in the downstream direction, forexample by raising the level of the bath above spillways in the fluidoutlet or by modifying the position of the spillways.

Inlet Area 1

The raw fluid to be treated, also known as effluent, is introduced intoeach reactor at the surface (as shown in FIG. 1) and across its entirewidth (via a spillway 1A, immersed or not, in practice formed by thelower edge of a slot defining the inlet in the wall of the reactor), orin an underflow arrangement (this variant is not shown), or at a singlepoint (at or under the surface).

Generally speaking, introduction at a single point may be selected inthe case of a single reactor or the first reactor of a series,communication between two successive reactors preferably being achievedby means of a spillway.

The inlet or admission area can be delimited from the remainder of thereactor by a siphon-shaped baffle 1B (or a jet breaker plate, achievinga siphon effect). This siphon-shaped baffle dissipates upstreamturbulent kinetic energy, essentially in the case of introduction at asingle point, and changes the flow direction (to vertical instead ofhorizontal).

A first reagent, such as a coagulant (for example a mineral salt such asan iron or aluminum salt) may be introduced into this highly turbulentarea in order to facilitate its dispersion. The manner of introducingthis reagent (or any other reagent) depends on how the raw fluid to betreated is introduced: single-point or multipoint injection over thewhole of the width of the admission area.

Central Area 2

This is the area of maximum turbulence inside the flow-guide in whichthe various components are intimately mixed.

Using a flow-guide means that a single agitator 8 can be used on eachshaft, regardless of the depth of the liquid.

The tube has an axis z-z of symmetry and preferably has a constant andadvantageously cylindrical section, which here is a circular section(i.e. it is cylindrical in the common sense of the term), but may haveother shapes, for example a polygonal shape.

This tube, which is completely immersed in the bath, is preferablyvertical (the axis z-z is therefore vertical), although inclined (oreven horizontal) directions are possible if the general configuration ofthe reactor as a whole, including its inlet/outlet, makes thisdesirable.

As shown in FIG. 4, a plurality of agitators equipped with flow-guidesmay be installed in the same vat, as a function of requirements and ifthe longitudinal and transverse dimensions of the vat allow it. Thenumber of agitators to be disposed in each direction (longitudinal andtransverse) depends on the ratio between the diameter of the mobiles ofthe agitators and the corresponding dimensions, the maximum diameteritself being a function of the depth of the liquid.

It is particularly advantageous if the agitator causes a verticaldownward flow.

The flowrate pumped by all of the agitators, which advantageouslyrepresents from 1 to 20 times the through-flowrate (that between theinlet and the outlet), is aspirated at the top of the tube 2A (ortubular sheath) and discharged towards the bottom of the reactor, andthus in a downward flow. This means that the reagent or reagents can beinjected at the surface, at a single point or through a perforatedannular tube, shown diagrammatically at 14, coaxial with the tube 2A anddisposed at the inlet thereof at its upper end. Here a flocculatingreagent is injected through this perforated tube, but it may instead beinjected at the inlet, between the inlet and the central area, or evenat the inlet of the tube.

The agitator 8 is advantageously disposed at the mid-height of the tube,which preferably has a diameter from 102% to 120% of that of theagitator.

The diameter is advantageously at most equal to 60% and at least equalto 40% of the largest horizontal dimension of the peripheral area, andis preferably of the order of 50% thereof. The hydraulic diameter of thecentral area is from 40% to 60% of the average dimension of the vat.

Its lower end advantageously faces the bottom at a distance from ⅓ to ⅔of the diameter of the tube, and the same applies to the distancebetween the surface and the level of the upper end of the tube. Thesedistances are advantageously of the order of 50% of the diameter of thetube.

The cruciform baffle 5 disposed under the flow-guide is formed of aplurality of vertical walls 5A extending from a common edge 7 that issubstantially aligned with the axis Z-Z of the agitator. This is astatic system to oppose continued rotation of the flow leaving theflow-guide tube; it channels the flow, divides it angularly into aplurality of equal parts, and prevents the formation of a circularcurrent at the bottom of the apparatus. Eliminating the radial effectthat is the cause of this rotation of the flow has the parallel effectof increasing the axial effect and thus of increasing the pumpedflowrate without increasing power consumption. This also preventsparticles accumulating in the corners formed by the cruciform baffle.

The cruciform baffle advantageously has a diameter from 75% to 125% ofthe diameter of the tube. Its walls extend over a significant fraction(at least ⅔) of the distance between the bottom of the vat and theoutlet of the tube, preferably over substantially the whole of thatdistance.

Here the cruciform baffle is formed of four walls joining at a rightangle (thus at 90%), two of which are preferably disposed transverselyto the direction of flow between the inlet and the outlet of the vat. Ina variant that is not shown the cruciform baffle may include a differentnumber of baffles, only three baffles or five baffles or even more.

Peripheral Area 3

Situated outside the flow-guide tube, this area is characterized by theupward movement of the pumped liquid (which has been well mixed but isstill only in the process of flocculation), low speed and turbulence andgood homogeneity, which means that the required process may be completedwithin a time period that has been minimized.

The disposition of the baffles 6A and 6B at the surface, transversely tothe through-flow, eliminates the rotational movement formed at thesurface and preferentially directs the whole of the incoming flow offluid towards the flow-guide tube, guaranteeing that the raw fluid to betreated passes through the flow-guide tube at least once in order to bemixed with the recirculated flow and the injected reagents.

The vertical baffles extend over only a portion of the height of thecentral area, and therefore of the height of the bath, preferably over40% to 60% of the height thereof and advantageously over the upper halfof the bath (especially when the inlet and the outlet are in the upperportion, as in the situation shown here). They advantageously extendbetween the surface and the level at which the agitator is disposedinside the tube.

In a variant that is not shown there may be a greater number of thesebaffles, for example three (or even four (or even more)) bafflesdisposed between the inlet and the outlet and angularly distributed in aregular or irregular manner.

Outlet Area 4

This area is opposite the inlet area and preferably at the same heightand advantageously includes a siphon-shaped baffle 4B and an immersedspillway 4A generally covering the whole of the width of the reactor.

Disposed at an optimum distance and at an appropriate immersion depth,by producing an appropriate upward speed, these devices minimizebypassing and dissipate turbulence that compromises treatment carriedout downstream of the reactor (for example if that treatment requiresphase separation).

Appropriate partitioning of the space between the siphon-shaped baffleand the downstream wall (not shown) ensures a flowrate that is evenlydistributed over the whole length of the spillway.

Specific Configurations

The reactor shown in FIGS. 1 and 2 is advantageously followed by aseparation area. Various configurations may be used.

FIG. 3 is a diagram representing the reactor 10 followed by a separationarea 100 of this kind.

It must be clearly understood that it is possible to connect in series aplurality of reactors like that of FIGS. 1 and 2, depending on thespecific requirements.

In FIG. 3, entry is from the left, downstream of a coagulating reagentinjection line 11, if any, and into a shaded area diagrammaticallyrepresenting the portion of the peripheral area in which upward flow isconfined angularly by the baffles 6A and 6B and in which mixing with theincoming raw fluid to be treated occurs. The diagram shows at 14 theinjection of flocculating reagent from a source 14A into the centralarea (not shaded) and at 15 the optional injection of powder from asource 15A of granular material. After flocculation, fluid leaves theright-hand portion of the peripheral area (shaded) and enters theseparation area, in which the flocculated fluid is separated intoclarified effluent and sludge containing the flocs formed in the reactor10.

The raw fluid to be treated is preferably water, which is in principlepre-coagulated by injecting a coagulating reagent such as an iron oraluminum salt via the line 11 or into an upstream vat fed by overflow orunderflow.

It is advantageous to inject powder to encourage the formation of flocsfrom impurities in suspension, colloidal impurities and dissolvedimpurities in the raw fluid to be treated. This material is preferably aballast consisting of a granular material that is insoluble (or onlyvery slightly soluble) in water and heavier than the raw fluid to betreated. This material is preferably sand, advantageously with aparticle size range from 20 to 300 microns.

FIG. 4 represents another reactor 101 including three flux-guide tubesin the same vat combined with a static system such as the cruciformbaffle shown FIGS. 1 and 2. The expression “flocculation area” refers tothe area formed conjointly by each central area defined by a tube andthe peripheral area around it, and the FIG. 4 reactor includes aplurality of flocculation areas disposed in parallel between an inlet(at the top in the drawing) and an outlet (at the bottom in the samedrawing).

FIG. 5 shows another reactor 10″ that is similar to that shown in FIGS.1 and 2 except that the wall of the vat is cylindrical and theflocculated fluid leaves the vat by overflowing along a large fractionof its periphery. Here there are four lateral deflector baffles 6″A to6″D delimiting four quarters of the upper portion of the peripheralarea, one of which (that at the top in the drawing) is reserved for theentry of raw fluid to be treated and the other three provide an outletfor the flocculated fluid.

Generally speaking, the flocs formed by the intensive mixing grow in theless agitated peripheral area and are partially recycled in theflow-guide before being finally transferred to the sedimentation area.

In the FIG. 6 embodiment, the sedimentation area is a separation vat100′ that is independent of the flocculation reactor 10 and includessedimentation assisting members 110 that here consist of inclined plates(in a different embodiment these members are omitted); finally, thesludge separated from the clarified effluent, which in practice leavesthe separation vat via a spillway, is here pumped by a pump 112connected by a line to a unit 113 capable of recovering a significantportion of the granular material contained in the sludge, in practice ahydrocyclone or any other suitable system capable of this separation, inwhich case the unit 113 is part of the powder source (the materialinjected into the flocculation reactor consists partly of this recoveredmaterial and partly of new material, of course).

The powder forms a ballast and is a completely inert material (forexample sand, garnet, etc.) or an active material (for example activatedcarbon powder or resin), which explains why it is sometimes listed inthe water treatment “reagents”, even if it is sand. As indicated above,it may be injected either at the inlet with the pre-coagulated fluid orin the area upstream of the agitation mobile, preferably at the top of aflow-guide coaxial with that mobile. Injecting ballast means that asingle vat with two effective areas can carry out, for example, theballasted flocculation treatment disclosed in French patents FR 2627704and FR 2719234, which corresponds to a process sometimes known as the“Actiflo” process, with or without sedimentation assistance plates,thereby reducing the overall implementation cost and minimizingagitation energy requirements. According to tests carried out by theApplicant, and going against the received wisdom of the person skilledin the art, the cruciform baffle of the present invention, by improvingagitation efficiency, minimizes the deposition of ballast sand on thebottom of the agitation vat.

FIG. 7 represents another installation for treating raw fluid, typicallywater. This installation differs from that shown in FIG. 6 essentiallyin that here the separation area is a vat 100″ around the flocculationvat 10, rather than downstream of the vat; the configuration of thereactor 10 is the same as in FIG. 6 and, as in FIG. 6, the powderinjected includes material recovered in the sludge pumped from thebottom of the vat 100″. In this embodiment there are no sedimentationassistance members, for example sedimentation assistance plates, but thelatter could be used.

In a variant that is not shown, the separation area uses a flotationprinciple, whereby the solids float on the surface of the clarifiedeffluent, rather than sedimentation, whereby the clarified effluent islighter than the sludge; separation assistance members may also be usedhere.

Tangential entry of the flocculated fluid into the separation area mayalso be advantageous, so as to benefit from a vortex effect in additionto the sedimentation effect.

Model-Based Comparative Study of the New Configuration and the StandardConfiguration

The advantages of the invention have been assessed by a study of thevelocity fields, turbulent gradient and treatment time distributionusing version 5 of the fluid mechanics simulation program Fluent fromFluent Inc.

Two configurations were studied, based on the same operating conditions,which differed as indicated hereinafter:

1—Highlighting the Advantages of a Surface Inlet-Outlet, a ShorterRotation Shaft and a Cruciform Baffle

Table 1 shows the advantages obtained on changing from a standardconfiguration A, with an underflow and/or overflow effluent inlet andoutlet, to a configuration B conforming to certain aspects of theinvention, namely: inlet and outlet at the same height, inclusion of ajet breaker plate and a siphon-shaped baffle, shorter rotation shaft andcruciform baffle placed at the bottom of the vat; note that the maximumand average values of the velocity fields and the turbulent gradient arehigher and more homogeneous with the new configuration B.

Benefits of the new configuration:

-   -   increased mixing efficiency for the same energy consumption;    -   maximum use of the vat volume;    -   reduced bypass;    -   facilitated series connection;    -   elimination of rotation shaft vibration problems as a result of        reducing its length;    -   reduced treatment time;    -   elimination of the risk of a preferred path in the vat by        creating a kinetic energy dissipation outlet area.

2—Highlighting the Advantages of Coupling the Cruciform Baffle,Flow-Guide and Chicane

Table 2 shows the advantages obtained, for continuous operation, onchanging from a standard configuration C with a single agitator to aconfiguration D with a vat equipped with a mobile, a cruciform baffle, aflow-guide resting on the cruciform baffle and two chicanes supportingthe flow-guide.

Analyzing the velocity fields and the turbulent gradient using theFluent fluid mechanics simulation software shows that, for the sameoperating conditions:

-   -   The maximum and average values of the velocity fields and the        turbulent gradient are higher and more homogeneous with the new        configuration D.    -   Two clearly separate areas are created: a highly agitated area        with very high values of the velocity and velocity gradient        inside the flow-guide and at the cruciform baffles and a less        agitated area outside the flow-guide.

Benefits of the new configuration:

-   -   improved mixing efficiency for the same energy consumption;    -   creation of two mixing areas differing in terms of mixing level,        enabling fast and efficient mixing of a reagent and the fluid in        the strongly agitated area and a circulation time in the less        agitated area that is the optimum for the reagent to act;    -   creation of a highly agitated area at the level of the cruciform        baffles, returning to suspension solid particles that have        settled out, or increasing the interface area in the case of a        gas-liquid flow;    -   reduced bypass;    -   reduced treatment time.

3—Highlighting the Advantage of Adding the Reagent by Toroidal InjectionAround the Flow-Guide (FIG. 3, Reference 14)

Graphical simulation of the distribution of a reagent shows thatdispersion is improved and faster with annular injection than withsingle-point injection.

TABLE 1 Improvement Parameter Ratio A to B for B Treatment time/average0.9  +9% treatment time Mixing time/average 0.79 +20% treatment timeMixing proportion after 0.62 +40% 1 minute Average velocity (m/s) 0.42+60% Average turbulent 0.21 +80% gradient (s⁻¹)

TABLE 2 Improvement Parameter Ratio C to D for D Treatment time/average0.956  +4% treatment time Mixing time/average 0.861 +14% treatment timeMixing proportion after 1    0%  15 minutes Average velocity (m/s⁻¹)0.17 +83% Average turbulent 0.044 +96% gradient (s⁻¹)

1. A method of treating water or wastewater in a system including areactor, a vertical flow channel disposed in the reactor, and aseparator, comprising: a. adding a flocculant to the water orwastewater; b. directing the water or wastewater into an open upper endof the flow channel immersed in the water or wastewater in the reactor,down through the flow channel, out a lower end of the flow channel andupwardly outside of the flow channel and through the reactor; c.continuously recycling the water or wastewater by directing the water orwastewater into the upper open end of the flow channel, downwardlythrough the flow channel, out the lower end of the flow channel,upwardly outside of the flow channel and through the reactor, and backinto the open upper end of the flow channel; d. mixing the flocculantand water or wastewater as the water or wastewater passes downwardlythrough the flow channel and wherein the mixing of the flocculant withthe water or wastewater forms a flocculated mixture including flocs; ande directing at least some of the flocculated mixture to the separatorand separating clarified effluent from sludge which contains the flocs.2. The method of claim 1 including continuously recirculating theflocculated mixture from the reactor downwardly through the immersedflow channel.
 3. The method of claim 1 wherein an agitator is disposedin the flow channel, and the method includes agitating the water orwastewater in the flow channel so as to give rise to an axial downwardflow of the water or wastewater.
 4. The method of claim 1 includingdividing the flow of the water or wastewater being discharged from thelower end of the flow channel.
 5. The method of claim 1 includingdirecting an inlet flow of water or wastewater into the reactor andmaintaining a flow rate through the flow channel greater than the flowrate of inlet water or wastewater.
 6. The method of claim 1 includingagitating the water or wastewater passing through the flow channel, andwherein the agitation within the flow channel is more intense thanagitation occurring outside the flow channel.
 7. The method of claim 1wherein the flow channel is disposed generally centrally within thereactor, and wherein the reactor includes an inlet and outlet disposedsuch that substantial portions of the water or wastewater to be treatedis required to be passed through the general central area of thereactor.
 8. The method of claim 1 including agitating the water orwastewater at generally a mid height in the flow channel resulting in adownward turbulent axial flow in the flow channel.
 9. The method ofclaim 1 including dividing the flow of the flocculated mixture beingdischarged from the lower end of the flow channel.
 10. The method ofclaim 9 including positioning a flow divider adjacent the outlet end ofthe flow channel for dividing the flow of the flocculated mixture. 11.The method of claim 1 wherein the reactor includes a surrounding wallstructure, and the flow channel is disposed inwardly and in spaced apartrelationship to at least a substantial portion of the surrounding wallstructure of the reactor, and wherein the bottom of the flow channel islocated from the bottom of the reactor a distance of approximately ⅓ to⅔ of the average width of the flow channel.
 12. The method of claim 11wherein the top of the flow channel is located a distance ofapproximately ⅓ to ⅔ of the average width of the flow channel under thesurface of the water or wastewater in the reactor.
 13. The method ofclaim 1 wherein the flow channel is disposed generally centrally withinthe reactor, and wherein the reactor includes an inlet and outletdisposed with respect to each other so as to generally require the wateror wastewater to be treated to move through the flow channel during thecourse of treatment.
 14. The method of claim 1 wherein the flocculatingagent is introduced upstream of the reactor or is introduced into thereactor.
 15. The method of claim 14 wherein the flocculant is introducedbetween an inlet to the reactor and the open upper end of the flowchannel.
 16. The method of claim 1 including introducing the flocculantinto a central area of the reactor.
 17. The method of claim 1 wherein atleast a portion of the flocculant is injected into the reactor generallycoaxially with the flow channel.
 18. The method of claim 1 includingmixing an insoluble granular material with the water or wastewater. 19.The method of claim 18 including mixing the coagulant with the water orwastewater to be treated.
 20. The method of claim 1 including mixing acoagulant with the water or wastewater to be treated.
 21. The method ofclaim 1 wherein the separator comprises a sedimentation tank and whereinthe flocs of the flocculated mixture settle in the sedimentation tank.22. The method of claim 1 including agitating the water or wastewaterwith an agitator in the flow channel to cause the flow to move axiallydown through the flow channel, and engaging the downward flowing wateror wastewater with a flow divider and dividing the flow after which thewater or wastewater moves upwardly in the reactor and outside of theflow channel.
 23. The method of claim 22 wherein agitation in the flowchannel causes the water or wastewater to be relatively more turbulentin the flow channel than outside the flow channel.
 24. The method ofclaim 1 wherein the flow channel is an elongated conduit having a wallstructure that constrains the water or wastewater entering the open topthereof to move downwardly through the entire conduit and to bedischarged out the lower open end thereof.
 25. The method of claim 1including an agitator disposed in the flow channel between the upper andlower ends and agitating the water or wastewater as the water orwastewater flows downwardly through the flow channel such that the flowof the water or wastewater through the flow channel is relatively moreturbulent than the flow of water or wastewater outside of the flowchannel; and engaging the water or wastewater in the vicinity of thelower end of the flow channel with a structure that causes the flow ofwater or wastewater leaving the flow channel and moving upwardly in thereactor to be less turbulent than in the flow channel.
 26. A method ofmixing a flocculant with water or wastewater in a water or wastewatertreatment system including a reactor and a vertical flow channeldisposed in the reactor, comprising: a. adding a flocculant to the wateror wastewater; b. directing the water or wastewater into an open upperend of the flow channel immersed in the water or wastewater in thereactor and moving the water or wastewater downwardly through the flowchannel, out the lower end of the flow channel, and upwardly outside theflow channel and through the reactor; c. as the water or wastewatermoves downwardly through the flow channel, mixing the flocculant withthe water or wastewater; d. wherein an agitator is disposed in the flowchannel between the upper and lower ends thereof, and the methodincludes driving the agitator and causing water or wastewater from thereactor to be induced into the open upper end of the flow channel and tobe moved downwardly therethrough such that the water or wastewater ismixed with the flocculant to form a water or wastewater mixture as thewater or wastewater moves downwardly through the flow channel; and e.directing at least some of the water or wastewater mixture to aseparator and clarifying the water or wastewater mixture.
 27. The methodof claim 26 including a flow divider disposed in the vicinity of thelower end of the flow channel and wherein the method includes dividingthe flow of water or wastewater being discharged from the lower end ofthe flow channel.
 28. The method of claim 26 wherein the agitator causesthe water or wastewater passing downwardly through the flow channel tobe at least slightly turbulent, and wherein the method entails engagingthe water or wastewater being discharged from the flow channel so as tocause the water or wastewater moving upwardly through the reactor andoutside of the flow channel to be less turbulent than the flow of wateror wastewater through the flow channel.
 29. The method of claim 28including continuously recirculating water or wastewater from thereactor into the open upper end of the flow channel and downwardlythrough the flow channel where the water or wastewater is dischargedtherefrom and thereafter moves upwardly in the reactor outside of theflow channel.
 30. The method of claim 26 wherein the agitator causes thewater or wastewater passing through the flow channel to become at leastslightly turbulent and to move downwardly in an axial flow pattern, andwherein the method entails engaging the flow of water or wastewaterbeing discharged from the flow channel and dividing the flow of water orwastewater being discharged from the flow channel.
 31. The method ofclaim 1, wherein both the flow channel and reactor include a horizontalwidth and wherein the horizontal width of the flow channel isapproximately 40% to approximately 60% of the horizontal width of thereactor.
 32. The method of claim 1, wherein the reactor includes aninlet and an outlet and wherein the flow rate of water or wastewaterpassing through the flow channel is approximately 1 to approximately 20times the flow rate of the water or wastewater passing out the outlet ofthe reactor.
 33. The method of claim 26 including continuouslyrecirculating the water or wastewater by directing the water orwastewater into the upper open end of the flow channel, downwardlythrough the flow channel, and out the lower end of the flow channel,upwardly outside of the flow channel and through the reactor, and backinto the open upper end of the flow channel.
 34. The method of claim 26,wherein the flow channel is disposed generally centrally within thereactor, and wherein the reactor includes an inlet and outlet disposedsuch that substantial portions of the water or wastewater to be mixedwith the flocculant is required to be passed through the general centralarea of the reactor.
 35. The method of claim 26 including agitating thewater or wastewater with the agitator in the flow channel to cause theflow to move axially down through the flow channel, and engaging thedownward flowing water or wastewater with a flow divider and dividingthe flow, after which the water or wastewater moves upwardly in thereactor and outside of the flow channel.
 36. The method of claim 26,wherein the mixing of the flocculant with the water or wastewater formsa flocculated mixture including flocs; and directing at least some ofthe flocculated mixture to a separator and separating clarified effluentfrom sludge which contains the flocs.
 37. The method of claim 36including dividing the flow of water or wastewater being discharged fromthe lower end of the flow channel.
 38. The method of claim 37 includingdirecting the water or wastewater through a flow divider disposedadjacent the lower end of the flow channel and dividing the flow of theflocculated mixture and directing at least a portion of the flocculatedmixture upwardly outside of the flow channel and through the reactor andback into the open upper end of the flow channel.
 39. A method oftreating water or wastewater in a system including a reactor, a verticalflow channel disposed in the reactor, and a separator, comprising: a.adding a flocculant to the water or wastewater; b. directing the wateror wastewater into an open upper end of the flow channel immersed in thewater or wastewater in the reactor, down through the flow channel, out alower end of the flow channel and upwardly outside of the flow channeland through the reactor; c. mixing the flocculant and water orwastewater as the water or wastewater passes downwardly through the flowchannel and wherein the mixing of the flocculant with the water orwastewater forms a flocculated mixture including flocs; d. agitating thewater or wastewater as the water or wastewater flows downwardly throughthe flow channel such that the flow of the water or wastewater throughthe flow channel is relatively more turbulent than the flow of water orwastewater outside of the flow channel, and engaging the water orwastewater in the vicinity of the lower end of the flow channel with astructure that causes the flow of water or wastewater leaving the flowchannel and moving upwardly in the reactor to be less turbulent than inthe flow channel, and recirculating the water or wastewater from thereactor into and downwardly through the flow channel; and e. directingat least some of the flocculated mixture to the separator and separatingclarified effluent from sludge which contains the flocs.
 40. The methodof claim 39 including continuously recirculating the flocculated mixturefrom the reactor downwardly through the immersed flow channel.
 41. Themethod of claim 39 including dividing the flow of the water orwastewater being discharged from the lower end of the flow channel. 42.The method of claim 39 including directing an inlet flow of water orwastewater into the reactor and maintaining a flow rate through the flowchannel greater than the flow rate of inlet water or wastewater.
 43. Themethod of claim 39 wherein the flow channel is disposed generallycentrally within the reactor, and wherein the reactor includes an inletand outlet disposed such that substantial portions of the water orwastewater to be treated is required to be passed through the generalcentral area of the reactor.
 44. The method of claim 39 includingpositioning a flow divider adjacent the outlet end of the flow channelfor dividing the flow of the flocculated mixture.
 45. The method ofclaim 39 wherein the reactor includes a surrounding wall structure, andthe flow channel is disposed inwardly and in spaced apart relationshipto at least a substantial portion of the surrounding wall structure ofthe reactor, and wherein the bottom of the flow channel is located fromthe bottom of the reactor a distance of approximately ⅓ to ⅔ of theaverage width of the flow channel.